Despite considerable progress in the treatment of leukemia, particularly in children, several subsets of leukemia remain highly resistant to treatment. Two types of resistant disease, acute leukemia in infants and epipodophyllotoxin-induced secondary leukemia are characterized by chromosomal translocations involving the MLL gene located at chromosome 11q23. In both cases, the reciprocal translocation results in the expression of a chimeric MLL fusion gene. The overall objective of this proposal is to test the hypothesis that a synthetic peptide that targets gene products uniquely expressed in these types of leukemia will selectively inhibit their growth. Findings presented herein indicate that the carboxy-terminus of the MLL fusion partner AF9 interacts with a small domain of another MLL fusion partner AF4. These portions of both AF9 and AF4 are present in leukemia-associated MLL fusion proteins suggesting that AF9 and AF4 are capable of interacting in their native form and/or as MLL fusion proteins. The physical interaction of MLL-AF4 with AF9 may be important in leukemogenesis in cells with t(4;11)(q21;q23) translocations characteristic of infant leukemia. A small synthetic peptide has been developed that disrupts the interaction of AF4 and AF9 in vitro. Furthermore, the peptide specifically inhibits proliferation of t(4;11) leukemia cell lines. Important to human health, this peptide- or derivative compounds- could provide a unique means of treating patients with infant leukemia or other leukemias with t(4;11) rearrangements. The more specific goals of this research project are to test the hypothesis that the peptide binds AF9 and disrupts its interaction with MLL-AF4 in t(4;11) leukemia cells. We predict that the peptide also binds AF9 in leukemia cells that lack t(4;11) rearrangements but data suggests that disruption of a native protein complex has little effect on these cells. Finally, we will perform an extensive mutational analysis of the AF4-AF9 protein interaction domains to determine the critical contact points that mediate binding to provide a basis for rational design of peptides to block AF4-AF9 binding. Ultimately, these studies may lead to the development of peptides or related compounds for the effective treatment of notoriously difficult human diseases including infant leukemia and treatment-related secondary leukemia.